SRVs

SRVs, in the context of the Domain Name System (DNS), refer to SRV records that specify the location of servers for specific services within a domain. An SRV record associates a service and a protocol with one or more target hosts and ports, enabling dynamic service discovery, load balancing, and failover without hardcoding server addresses. They are defined to support scalable, location-aware client connections in networks and Internet services.

The canonical form of an SRV record’s name is the service and protocol, for example _sip._tcp.example.com. The RDATA contains four 16-bit fields: priority, weight, port, and target. The target is a domain name that has A or AAAA records. Lower priority values are tried first; among records with the same priority, the weight field biases selection, providing simple load balancing across multiple servers.

SRV records are used by a variety of services, including SIP for VoIP, XMPP/Jabber for instant messaging, and directory services in enterprise networks. In Windows environments, DNS SRV records play a key role in locating domain controllers and other services, with records like _ldap._tcp.dc._msdcs.<forest> guiding clients to LDAP servers.

Querying and resolution involve DNS lookups that return the set of possible targets, after which the client selects a server based on priority and weight. SRV records support service resilience by allowing multiple endpoints to be available for the same service and domain.

Limitations include the need for correct DNS configuration and the fact that not all applications support SRV-based discovery. Caching and TTL settings can affect responsiveness to topology changes. The standard defining SRV records is RFC 2782, published in 2000.